We plan to continue our investigation of the effect of vitamin D status on 25-OHD3-1 alpha-hydroxylase and its responsiveness to PTH in the normal and near normal metabolic circumstance, as judged in the chick by normal somatic growth, normocalcemic and normophosphatemic. In chicks, we will administer vitamin D in doses ranging from 15 to 60 iu daily from hatching to three weeks of age, (have confirmed that this range of vitamine D promotes normal growth and maintains normocalcemia and normophosphatemia). At three weeks, we will determine 25-OHD3-1 alpha-hydroxylase activity, its response to administration of PTE, and measure circulating plasma levels of 25-OHD, 1,25-(OH)2D, both of which we measure in our laboratory. We are also in the process of comparing 25-OHD3-1 alpha-hydroxylase activity, and circulating concentrations of 25-OHD and 1,25-(OH)2D levels in the vitamin D-replete chick, with those in the minimally vitamin D-deprived chick, again in the demonstrated circumstance if normal somatic growth and normocalcemic and normophosphatemic. We plan to complete studies in the vitamin D deficient rooster in which we have demonstrated for the first time (in the chicken) that the renal tubular function is impaired and to some extent correctable with acute administration of 1,25-(OH)2D3. We plan to pursue the observation that in children with moderate renal insufficiency (glomerular filtration rate, 41 plus or minus 3 ml/min/1.73M2) the plasma level of 1,25(OH)2D3 is reduced, on average, by a third, in contrast to the normal levels reported in adult patients with similar degree of renal insufficiency.